By the wording "the one lab bench", the writer of the article emphasizes the smallness of Fleming's lab, prefatory to describing the circumstances that led to a colony of Penicillium notatum mold in a staphylococci-culture dish:

Fleming was ... given a basement laboratory tucked in next to the boiler room. ... Small dishes of deadly staphylococci, streptococci, and pneumococci bacteria were lined and labeled across the one lab bench that stretched the length of Fleming's lab. Molds were the one great hazard to Fleming's lab operation. Fleming's lab alternated between being drafty and stuffy, depending on the weather and how hard the boiler worked next door. His only ventilation was a pair of windows that opened at ground level ... Afternoon breezes blew leaves, dust, and a great variety of airborne molds through those windows.

The serendipity of a Penicillium mold landing in a culture dish is often made much of in descriptions of Fleming's discovery; in this case, the author makes the scene and circumstances more vivid by including details of laboratory layout, and perhaps hinting that if there had been more lab benches, with some away from the windows, the critical event would have been less likely to happen.